The use of semiconductor diode lasers (hereinafter referred to simply as diode lasers) for forming a source of optical energy is attractive for a number of reasons. For example, diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Also by example, the diode laser is a monolithic device, and does not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output beam.
One disadvantage of the semiconductor diode laser is the relatively low power of the output beam, as compared to other types of laser devices.
In an attempt to increase the output power, while still preserving the advantages of the diode laser, researchers have combined a plurality of individual semiconductor diode lasers into arrays. In this approach the total output power becomes a function of the number of individual diode lasers that comprise the array.
One such approach is referred to in the art as a `rack and stack` configuration. In the rack and stack configuration a bar comprised of a plurality of side-by-side edge emitting Fabry-Perot diode lasers is sandwiched between two electrically conductive buses that provide DC operating power to the laser diodes of the bar. A number of bars can be vertically stacked in this manner, yielding a two dimensional array of individual Fabry-Perot diode lasers.
A related approach is referred to in the art as `bars and grooves`, wherein an electrically insulating substrate has a plurality of parallel grooves into which are inserted bars comprised of the plurality of side-by-side edge emitting Fabry-Perot diode lasers. The bars can be soldered into place and thus both mechanically stabilized and electrically connected to a power bus. U.S. Pat. No. 5,040,187, entitled "Monolithic Laser Diode Array" by A. Karpinski, is representative of this approach.
One problem that arises in either of these approaches is the use of the conventional edge cleaved and edge emitting Fabry-Perot type of diode laser. This type of diode laser exhibits severe alignment constraints (i.e., exhibits submicron tolerances) when it is desired to optically couple the array to an external lens, such as a collimating lens. Similar problems exist when using monolithic arrays of 45.degree. mirror type surface emitting lasers.
A more attractive type of diode laser is known as the vertical or surface emitting (SE) type, wherein the output beam is emitted through a window from the `top` of the diode laser, as opposed to being emitted from a cleaved edge as in the Fabry-Perot. Distributed feedback (DFB) can be employed within the diode laser to provide a resonant cavity structure. The following U.S. Patents are all representative of surface emitting DFB (SE-DFB) diode lasers: U.S. Pat. No. 5,345,466 (Sep. 6, 1994) entitled "Curved Grating Surface-Emitting Distributed Feedback Laser" by S. H. Macomber (an inventor of this patent application); U.S. Pat. No. 5,241,556 (Aug. 31, 1993) entitled "Chirped Grating Surface-Emitting Distributed Feedback Semiconductor Lasers" by S. H. Macomber and J. S. Mott; and U.S. Pat. No. 5,238,531 (Aug. 24, 1993) entitled "Apparatus and Method for Fabricating a Chirped Grating in a Surface-Emitting Distributed Feedback Semiconductor Laser Diode Device by S. H. Macomber and J. S. Mott. Reference may also be had to following publication: S. H. Macomber et al., "Surface-emitting distributed feedback semiconductor laser", Appl. Phys. Lett., vol. 51, pp.472-474, 1987.
Of particular interest herein are the following two publications, both of which were coauthored by an inventor of this patent application. These two publications report on arrays of SE-DFB diode lasers: S. H. Macomber et al., "Recent developments in surface emitting distributed feedback arrays", Proc. SPIE, vol. 1219, pp. 228-232, 1990; and J. S. Mott et al., "Two-dimensional surface emitting distributed feedback laser arrays", IEEE Photon. Lett., vol. 1, pp. 202-204, 1989. Both of these publications describe the mechanical and electrical combination of a chip containing a plurality of SE-DFB diode lasers with a water cooled copper/silicon microchannel supporting structure and heat sink.
One problem presented by conventional approaches to array fabrication is a difficulty in providing an array of SE-DFB diode lasers that are prescreened and burned-in prior to being incorporated into the array.
In this regard it can be shown that if y is the probability that a single element of a monolithic array of diode lasers will operate to specification, then the probability P that an N-element monolithic array will have all diode lasers operating to specification is P=y.sup.N. For example, in the case of a five element array of diode lasers, and assuming a typical process yield of y=0.5, the array yield is approximately only 3%. It can be appreciated that the probability of fabricating a totally functional larger array (e.g., a 64 element array) is essentially zero.
Another problem presented by conventional approaches is a difficulty in providing an array of SE-DFB diode lasers that can be individually selected and operated independently of the other diode lasers of the array. A further problem presented by conventional approaches is a difficulty in providing an array of SE-DFB diode lasers that can be individually selected and operated independently of the other diode lasers of the array, and which furthermore can have different output wavelengths.
A still further problem presented by conventional approaches is providing a voltage adding type of array, wherein individual ones of the diode lasers of the array are connected together in series as opposed to being connected in parallel. The provision of a series connected, `voltage adding` array greatly simplifies the cabling and power supply requirements over a parallel connected array. For example, if each diode laser requires approximately 10 amperes at two volts to operate; a 64 element parallel connected array will require as a minimum a two volt, 640 ampere power supply. However, a 64 element series connected array will require instead, as a minimum, a 128 volt, 10 ampere power supply. It can be appreciated that at least the power supply and cabling for the latter configuration is significantly less of a problem to implement than the power supply and cabling for the former, parallel-connected embodiment.